drm/i915: Switch planes from transitional helpers to full atomic helpers

There are two sets of helper functions provided by the DRM core that can
implement the .update_plane() and .disable_plane() hooks in terms of a
driver's atomic entrypoints.  The transitional helpers (which we have
been using so far) create a plane state and then use the plane's atomic
entrypoints to perform the atomic begin/check/prepare/commit/finish
sequence on that single plane only.  The full atomic helpers create a
top-level atomic state (which is capable of holding multiple object
states for planes, crtc's, and/or connectors) and then passes the
top-level atomic state through the full "atomic modeset" pipeline.

Switching from the transitional to full helpers here shouldn't result in
any functional change, but will enable us to exercise/test more of the
internal atomic pipeline with the legacy API's used by existing
applications.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>